Scosche makes NN03RB (connectors which plug into the HU) and NN03 (I think that is the number) plugs into the factory wiring harness. You solder the appropriate wires to your LOC and to make a jumper from the NN03RB to the NN03 to provide power, etc. to the HU, and then you plug it together.
Check the photo in post #13 in this thread:
RF speaker upgrade
That is the RF HU, but I think the same would apply to the midline.
And then you would have to make connections from your amp(s) to the speaker wires (or pull new wires).
I don't think you need a six channel LOC as you don't have six channels out of the HU. I used a 4 x 100 amp connected to a 4 channel LOC, and then low-passed the signal thru put from the 4x100 amp to the sub through a second amp.